The present invention relates to a coil pattern of a frequency generator which provides a frequency signal proportionally to the revolution speed of a motor.
In general, the structure of a conventional drum motors used in a VTR(Video Tape Recorder) or a DAT(Digital Audio Tape) comprises a core 4 wound with a coil 3 which is mounted on the upper side of a base 2 fixed to printed circuit boards PCB 1 and 1', a rotor case 6 with a magnet 5 which is installed at the outer side of the core 4, and a shaft 8 which is installed at the center of the rotor case 6, being supported by a bearing 7.
Thus, the rotor case 6 is revolved by the interaction of the magnet 5 in the rotor case 6 with the coil 3, if the current flows through the coil 3 wound around the core 4. On the other hand, the PCB 1a is provided with a driving circuit element mounting part 10 and the PCB 1 for a coil pattern of frequency generation is mounted on this PCB 1a.
As shown in FIG. 2 (A), a coil pattern of rectangular teeth type is formed around the PCB 1 and has contact points (e) and (f) thereon through 360.degree.. Also, the lower side of the magnet 5 in the rotor case 6 is facing the coil pattern and provided with magnetic poles N,S,N,S, by the same pich to that of the coil pattern of the PCB 1, as shown in FIG. 2(B). (The magnet 5 is shown in an unfolded rectangular form.) Thus, a frequency signal which is proportional to the revolution speed of a motor 9 is detected when the magnet 5 is revolved according to the revolution of the rotor 6 for controlling the speed of the motor 9.
In this case, however, the PCB 1 provided with the coil pattern for frequency generation and the other PCB 1 for driving the motor 9 should be applied separately each other or with an expensive dual pattern PCB due to their size and insufficient space. Thus, the number of required components and the cost are increased and the assembling process thereof is complicated. Moreover, in the magnetic-changed frequency generation method, there is a problem that magnetic-induced noise is generated from an external magnetic field produced from a motor, a transformer and the like.
Accordingly, in order to remove this magnetic-induced noise of the frequency generating coil from the PCB to therearound, an additional coil for removing the magnetic-induced noise is provided in the opposite direction at both sides of the frequency generating coil. However, such an magnetic induced noise cannot be removed completely, and the voltage of the magnetic-induced noise has a bad effect on the revolution accuracy of the frequency generator, when the voltage of the generated frequency signal is low.